In the context of offshore hydrocarbon production, a riser is a tubular conduit that extends from a subsea structure to a production platform, for the purpose of conveying produced hydrocarbons from the well to the platform. The subsea structure may be a “christmas tree”, riser base, Blowout Preventer (BOP), or some other structure.
Risers are configured in order to allow for movements of the platform relative to the subsea structure in both a horizontal and a vertical direction, for example to facilitate drilling and well maintenance operations to be performed from the platform and to accommodate the effects of ocean currents, wind and waves on the risers and platform. Riser configurations include a “free hanging” configuration, a Reverse Pliant Wave configuration, a Pliant Wave configuration, a lazy S configuration, and other suitable configurations.
Offshore hydrocarbon production platforms may be fixed or floating. In addition to collecting produced hydrocarbons, floating production units (FPUs, a term that encompasses floating offshore hydrocarbon production platforms such as semisubmersible floating production systems; ship-shaped floating production, storage and offloading systems; tension leg platforms; and spar platforms) may also be used as drilling platforms to drill multiple new subsea wells. The subsea wells may be located substantially directly underneath the FPU, with the risers connecting the FPU to the well, and the drilling apparatus may extend downwards from the centre of the FPU. It may therefore be necessary to move the FPU in order to allow new wells to be drilled and/or to allow existing wells to be maintained. The connection between the platform and the subsea structures via the risers can be maintained during drilling and during the movement of the platform, thus allowing hydrocarbon production and collection to continue. By way of example, an FPU may be moveable horizontally by a distance in the region of 80 meters in any direction relative to a central position above the subsea wells. Movement is achieved by pulling in and playing out mooring lines attached to the four corners of the FPU.
For riser removal and installation operations on platforms where the risers are exposed/hanging on the platform side, the normal methodology involves the use of an installation vessel (IV) in addition to the platform itself. The IV is equipped with industry-standard laying spread (normally Vertical Laying System—VLS) and reels/carousels/baskets for storage of risers. The IV can install the riser from both directions, i.e. either the subsea or the topside end of the riser may be installed first (subject for example to already in-place riser configuration, auxiliary equipment size, packing constraints etc.).
FIGS. 1 to 6 illustrate schematically various stages in the conventional riser installation process in the case of a Floating Production Unit (FPU) where the riser is being installed to achieve a Pliant Wave configuration. Very generally, the process involves the following steps:
(1) An end of a pull-in wire, suspended from the platform, is transferred from the platform to the installation vessel (FIG. 1).
(2) The transferred end of the pull-in wire is attached to the bottom of the riser that is being constructed at the installation vessel, such that the pull-in wire is now suspended between the platform and the riser. This bottom end of the riser will subsequently be attached to the subsea structure and so is referred to hereinafter as the “subsea” end of the riser.
(3) The riser is fed into the water from the installation vessel and an increasing length of the riser is laid along the seabed. During this stage, the installation vessel steps away from the platform.
(4) At this stage, the riser has adopted a flat-bottomed U-shaped configuration in the water (FIG. 2). At some intermediate point in this procedure clump weight(s) and anchor clamp(s) are installed at predetermined points on the riser and buoyancy modules are installed around the riser, over a certain length.
(5) As the riser is deployed further the clump weight comes to rest on the seabed. The riser is then tethered to a preinstalled subsea anchor by attaching the anchor clamp to the subsea anchor. The installation vessel then moves back towards the platform, deploying the riser in the Pliant Wave configuration (FIG. 3).
(6) The subsea end of the riser is transferred from the pull-in wire to a static wire depending from the platform (FIG. 4).
(7) The pull-in wire is transferred to the installation vessel and a second end of the riser, which will subsequently be attached to the platform and so is referred to hereinafter as the “topside” end of the riser, is attached to the pull-in wire. The topside end of the riser is also attached to an abandonment and recovery (A&R) wire depending from the installation vessel. The topside end of the riser is lowered from the installation vessel using the A&R wire until the pull-in wire is taking the weight of the riser. The A&R wire is then disconnected from the topside end of the riser, and the topside end of the riser is pulled in to the platform using the pull-in wire and is then attached to the platform at the platform's riser balcony (FIGS. 4 and 5).
(8) The installation vessel moves to the side of the platform opposite the riser balcony, and a crane wire is lowered from the installation vessel and connected to the subsea end of the riser, typically using an ROV (FIG. 5).
(9) The subsea end of the riser is disconnected from the static line depending from the platform and is lowered to the seabed in the vicinity of a subsea structure using the crane wire from the installation vessel. The subsea end of the riser may be connected to the subsea structure using an appropriate tie-in tool and/or divers and/or an ROV (FIG. 6).
WO2011/099869 describes a method of installing a riser, similar to the method described above.
When implementing riser installation using these known procedures, it is necessary for the installation vessel to move close in to the platform's riser balcony (typically 20-40 m away) during the transfer of the subsea end of the riser and the topside end of the riser to the platform (FIG. 1), in order to maintain the integrity of the guide tube loads, maintain the bending radius of the riser, and prevent any contact between the riser and the port in the installation vessel through which it is lowered. The fact that the installation vessel must move so close to the platform is a significant disadvantage due to the risk of collision with the risers hanging from the riser balcony. Indeed, many operators/regulators define an exclusion zone in the area adjacent to the riser balcony, e.g. typically extending up to 200 m away from the riser balcony. If vessels do need to enter the exclusion zone, production and other operations must be ceased. Any cessation of production represents a significant expense.
It will be appreciated that risers may be removed using the reverse of the installation procedure described above (FIGS. 1 to 6) and that similar problems may be encountered.
It is noted that procedures similar to those used to install and remove risers may be used to install and remove other types of flexible lines, such as power cables and umbilicals. As such, the same problem is encountered, i.e. the need for an installation vessel to enter the exclusion zone and consequential shut down of operations.